3D anisotropic P-wave tomography of the Central Mediterranean: new insights into slab geometry and upper mantle flow patterns

Characterized by the coexistence of different compressional and extensional phases associated with episodes of orogenesis, slab rollback, slab tearing and oceanic spreading, the Central Mediterranean represents one of the most interesting convergent margin on Earth. Since the late 1990s, several seismologists have studied this region aiming at imagining the isotropic and anisotropic structures below its surface. Although numerous researchers have demonstrated that performing P-wave tomography neglecting seismic anisotropy can introduce significant imaging artefacts, prior tomographic studies have largely assumed an isotropic Earth. Using the method proposed by VanderBeek & Faccenda (2021), here we discard the isotropic approximation and invert for both P-wave isotropic velocity anomalies and seismic anisotropy and present the first 3D anisotropic P-wave tomography of the upper mantle covering the entire Central Mediterranean. Our results show that inverting for seismic anisotropy strongly reduces the magnitude of the isotropic P-wave anomalies. This suggests that lateral variations in temperature and/or composition are smaller that what can be inferred from purely isotropic tomographies. P-wave fast azimuths orient mostly parallel to the trend of the Balcanic and the Alpine orogens in Eastern and Central Europe, respectively. In the Central Mediterranean the P-wave fast azimuths are sub-parallel to the Oligocene/Miocene-to-present retreating direction of the Ionian trench which led to the opening of the Liguro-Provençal and Thyrrenian basins and rotation of the Corsica-Sardinia block. We find that the pattern of the P-wave fast azimuths is largely consistent with the S-wave fast azimuths determined from the splitting of SKS waves and from Rayleigh waves. This poses further constraints on the interpretation of the regional geodynamic evolution and on the accuracy of the employed inverse method.

References:

VanderBeek, B. P., & Faccenda, M. 2021. Imaging upper mantle anisotropy with teleseismic P-wave delays: insights from tomographic reconstructions of subduction simulations. Geophysical Journal International,225(3), 2097–2119.